Scientists and engineers have developed a number of devices that exploit the effects of catalytic reactions. For example, today there are catalytic monitors designed to measure the concentration of organic gases in air, and to detect the existence of hazardous conditions. These monitors can reduce the risk of accident by detecting the presence of explosive gases which can build up or be released at public utilities, propane distributors, fire services, HVAC contractors, landfill operators, steel mills, natural gas buses and other similar locations. These instruments typically include an element that is generally referred to as a pellistor or pelement. A typical pellistor consists of a small ceramic bead cast on a coil of wire, where the wire serves as both a heater and a thermometer. Electronic circuitry determines the resistance of the element and hence he pellistor's temperature rise or the decrease in power required to maintain the pellistor at a constant temperature when exposed to a gas containing a combustible constituent. In effect, the heat of oxidation of the analyte is measured and, through a calibration procedure, related to the quantity of the analyte present in the gas stream. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987).
Pellistor catalysts usually contain palladium, platinum, or a mixture or alloy that includes at least one of these two noble metals. Palladium catalyzes methane combustion in air at a lower temperature than platinum. Platinum, when maintained at a temperature sufficient to combust methane, is less susceptible than palladium to poisoning by sulfur. These characteristics have resulted in palladium sensors being preferred for battery operated equipment where power consumption must be minimized and platinum sensors being preferred for fixed gas detention systems where long life is desirable.
The chemical process catalyzed by the pellistor is the oxidation of an organic gas in air to yield mostly water vapor and carbon dioxide. Although such catalyst systems can work well initially, deposits can build up and remain on the catalyst surface and, in time, decrease the pellistor's sensitivity. Such deposits may form if the gas contains molecules or atoms that are not readily converted to vapors by oxidation. Such molecules or atoms are referred to as catalyst poisons. Commonly encountered poisons for palladium include sulfur-containing organic compounds such as odorants, organohalides, organosilicons, organoleads, and organophosphates. Typically, only organosilicons, organoleads, and organophosphates act to poison pellistors catalyzed by platinum. To address the problem of catalyst poisons, a filter may be incorporated around or on the pellistor. Such pellistors can absorb a finite amount of poison and may have a some-what extended life, but will be poisoned by repeated or high level exposures.
Once poisoned, the catalyst is generally ineffective in detecting the presence of combustible gases. This renders the catalytic device unusable. Consequently, catalytic poisoning is a costly problem as it can destroy the usefulness of expensive catalyst system such as gas detectors, as well as systems for converting conversion or reforming of petroleum feedstocks into other chemical compounds and catalytic air cleaning systems for automotive exhaust gases, which can lose activity due to exposure to the oxidation resistant poisons often contained in engine lubricants. For example, W. H. Preston et al. in the Institution of Mechanical Engineers Papers, Conference on Vehicle Emissions and Their Impact on European Air Quality, 1987-88, has shown a statistical link between the phosphorous content of lubricants and the catalyst performance of automobile air cleaning systems.
Further troubling is that poisoning from metalloids such as boron, silicon, germanium, arsencic, and antimony can resist existing recovery techniques. Specifically, metalloid-poisons form polymeric oxides which are not converted to gases by heating in oxygenated environments. Consequently, although both non-metal and metalloid-containing organic compounds poison noble metal catalysts, the polymeric metalloid oxides cannot be removed by oxidation. Accordingly, there is need for a recovery process that can treat catalysts poisoned by metalloid compounds.